invention relates to a television signal recording and reproducing system employing an optical recording medium for recording thereon and reproducing therefrom a television signal which is an analog image signal.
For recording and reproducing of an analog image signal which has a wide band characteristic, such as a television signal (hereinafter referred to as a TV signal), a magnetic tape is generally used as in a video tape recorder (VTR). In recent years, however, there has been developed a recording and reproducing system which utilizes an optical disc, opto-magnetic disc, or similar optical recording medium, and the system has already been put to practical use in an optical disc (a laser disc) exclusively for reproducing.
The recording of a TV signal by the use of such a frequency modulation system , which uses an FM wave frequency-modulated with the TV signal, usually calls for a frequency band twice that of the TV signal. On this account, for recording the TV signal on a supplementable or reloadable optical disc, it is necessary that the optical disc possess wide band performance which permits recording of high frequency components as well. Further, a physically high precision readout device is needed for reading out recorded pits.
In general, according to the conventional frequency modulation system (hereinafter referred to as a "positive modulation system"), as the frequency increases, the pit length to be recorded decreases and the recording energy decreases. Moreover, the accuracy of the pit length lowers in relation to the pit length and the SN ratio of the TV signal (ratio of signal to noise of TV signal before frequency modulation) drops. Besides, during reproducing of pits fluctuations of the readout output, which occur at pit ends, cause a rapid decrease in the CN ratio of the FM wave (ratio of carrier power of an FM wave frequency-modulated by a TV signal to noise power in a frequency band of the FM wave) with an increase in frequency.
On the other hand, optical recording media now available are the afore-mentioned laser disc for playback only and optical discs of supplementable or reloadable type. The laser disc is advantageous in that the SN ratio in reproducing is excellent and that the same recorded information can be produced in large quantities, since recorded information is physically written on the disc, but the laser disc is defective in that recorded information cannot be erased nor can new information be written. In contrast thereto, the optical disc permits recording and reproducing of information and enables recording of a large quantity of information but has a shortcoming that the SN ratio in reproduction is lower than that of the laser disc. "Accordingly, even if the frequency modulation system now employed in the laser disc is applied to the optical disc, it is difficult to obtain a sufficiently excellent reproduced signal." Although there has been strong demand for a recording and reproducing system which would not incur any significant degradation of the picture quality even if applied to the optical disc, such a system has not yet been proposed at all.